Adverbs
In English, adverbs are usually formed by adding -ly to the end of an adjective. In the comparative, more (or less) is added before the adverb, as in "This sofa seats three people more comfortably than the other one." Some irregular adverbs such as fast or hard do not use more, but add an -er suffix, as the adjectives do. Thus: "My new car starts faster than the old one" or "She studies harder than her sister does."
For some one-syllable adjectives, the comparative of adjectives may be used interchangeably with the comparative of adverbs, with no change in meaning: "My new car starts more quickly than the old one" or "My new car starts quicker than the old one".
However, if the adjective has an irregular comparative, then the adverb must use it: "She writes better than I do" or "He threw the ball farther than his brother did."
Read more about this topic: Comparative